The invention relates to a liquid level detecting apparatus, and more particularly to a liquid level detecting apparatus for automatically detecting a residual amount of a liquid stored in a fuel tank for transport means, such as automobiles and airplanes, by detecting a liquid level thereof.
There are previously known a liquid level detecting apparatus for detecting a liquid level in a fuel tank, for example, of automobiles, in which a float moves upward and downward depending on the liquid level such that a floating arm is slid on a resistance plate, and thus the liquid level is converted into an electrical potential difference, thereby detecting the liquid level.
Herein, an example of the liquid level detecting apparatus will be described. FIG. 1 is an electric block diagram illustrating a configuration of a sensor used in the liquid level detecting apparatus according to the invention and the related art. FIG. 2 is a diagram illustrating a configuration of the liquid level detecting apparatus according to the invention and the related art. FIG. 3 is a diagram illustrating a configuration of a variable resistor in the sensor according to the invention and the related art.
The sensor 2 of the liquid level detecting apparatus 1 includes a variable resistor 3 which changes a resistance value by allowing contacts 19 and 20, as described below, to move in association with a change in liquid level within a hermetically sealed vessel inside T. The variable resistor 3 is connected in series to a fixed resistor 7, and in turn connected to a power supply circuit 4, which applies a predetermined voltage to the variable resistor 3 and the fixed resistor 7
The sensor 2, as shown FIGS. 2 and 3, includes a resistance plate 13 attached to a body frame 12, and a sliding contact element 14 connected to a proximal end of a floating arm 11, which has also a distal end attached to a float 10 configured to float on a surface of a liquid by buoyancy relative to the liquid. The resistance plate 13 of the sensor 2 is provided with a first conductive pattern 15 and a second conductive pattern 16. These first and second conductive patterns 15 and 16 are arranged in parallel to each other in an arc-like shape about a rotational axis 21 of the floating arm 11. An input/output conductive portion 17 is connected to one end of the first conductive pattern 15, and an input/output conductive portion 18 is connected to one end of the second conductive pattern 16.
The first conductive pattern 15 consists of a plurality of elongated conductive segments 15a arranged in a circumferential direction of the arc-like shape at a predetermined interval and a resistance element 15b electrically connecting the conductive segments 15a to each other. Also, the second conductive pattern 16 consists of a plurality of elongated conductive segments 16a arranged in a circumferential direction of the arc-like shape at a predetermined interval and a connecting element 16b electrically connecting the conductive segments 16a to each other.
The sliding contact element 14 is provided with the contacts 19 and 20 electrically connected to each other. Also, the rotational axis 21 located on the proximal end of the floating arm 11 is connected to the sliding contact element 14. The floating arm 11 pivots about the rotational axis 21 as a supporting point in a arrow Y direction in FIG. 3 by allowing the float 10, which floats on the surface of the liquid, to downwardly move according to an amount of the liquid consumed from a liquid level in full tank condition. In response to such a pivoting of the floating arm 11, the sliding contact element 14 also rotates in the arrow Y direction in FIG. 3. By such a rotation of the sliding contact element 14, each of the contacts 19 and 20 slide on and contact electrically with each of the conductive segments 15a and 16a respectively disposed on the first conductive pattern 15 and the second conductive pattern 16. As a result, a length of the resistance element 15b interposed in a circuit between the input/output conductive portion 17 connected to the first conductive pattern 15 and the input/output conductive portion 18 connected to the second conductive pattern 16 is changed, and thus a resistance value of the circuit is changed (i.e., the resistance value of the variable resistor 3 in FIG. 1 is changed). As described above, the variable resistor 3 consists of the first conductive pattern 15, the second conductive pattern 16, and the sliding contact element 14.
An electrical potential difference between the input/output conductive portions 17 and 18 caused when a voltage is applied to the variable resistor 3 is detected by the sensor 2, and an output signal of the sensor 2 is sent to a processing circuit 5. Then, the processing circuit 5 displays a residual amount of the liquid based on the output signal of the sensor 2 on an indication device, such as a gauge 6, in an analogue or bar graph manner. Meanwhile, a fixed resistor may be disposed in the gauge 6 on a wire for connecting the gauge 6 to the processing circuit 5.
In such a liquid level detecting apparatus, silver-palladium (AgPd) alloy, silver-copper (AgCu) alloy, silver-nickel (AgNi) alloy, and the like are generally used as the material of the contacts. The conductive segments are made of a mixture of silver-palladium (AgPd) powder and glass, for example, and are manufactured by mixing silver powder, palladium powder, and glass powder to form a paste, printing the obtained paste on the resistance plate, drying, and then sintering.
However, the liquid level detecting apparatus can be used in a fuel tank of an automobile using as a fuel an electrolyte (alcohol) itself, such as ethanol and methanol, or gasoline containing such an electrolyte. Silver (Ag) has a lower electrical resistance and an excellent conductivity, but the contacts and the conductive segments containing such silver can be deteriorated or eroded by a sulfur component, water, an alcohol component and the like in the fuel, thereby causing an impediment in which measuring cannot be performed or an incorrect value can be created, etc., due to a poor electrical conduction. Also, due to a present world fuel situation, a possibility of using various mixed fuels is being increased, and thus, it is necessary to provide a reliable fuel system which can prevent such an impediment. Therefore, to prevent the deterioration and erosion of the conductive segments and the contacts, there are known technologies in which portions of the conductive segments, on which the contacts are slid, are coated with an alloy containing gold (Au)(e.g., see Patent Documents 1 and 2).
Patent Document 1: JP-A-2003-287456
Patent Document 2: JP-A-2009-162694
The technologies according to the Patent Documents 1 and 2 has an effect to deterioration and erosion resistances of the conductive segments, but tend not to provide an sufficient effectiveness of the effect because the coated layer can be thinned over time. In addition, to obtain a sufficient deterioration and erosion resistances, it is necessary to contain a large amount of gold (e.g., approximately 40% mass or more in case of the conductive segments), thereby causing a problem of increasing costs.